This demonstrates density. Fill a bottle with water. Add a thin later of oil. Add a few drops of food coloring. Sprinkle with salt. The salt is more dense than the oil so is moves through it carrying the color and a drop of oil with it. The oil bobs back up to the top. I got this idea from http://www.kiwicrate.com/projects/Salt-Volcano/2331

This year I am planning on having the kids make blocks from milk cartons. We will stuff them with paper from the recycle bin. Since we have about 20 kids and drink milk every day we will end up with a lot of blocks!